Method for glossing articles made of aluminum and particularly pure aluminum and its alloys



2,756,131 Patented July 24, 1956 METHOD FOR GLOSSING ARTICLES MADE OFALUMINUM AND PARTICULARLY PURE ALU- MINUM AND ITS ALLOYS HeinrichNeunzig, Gindorf, near Grevenbroich, Germany,

assignor to Vereinigte Aluminium-Works: Aktiengesellschaft, Bonn(Rhine), Germany, a corporation of Germany No Drawing. ApplicationAugust 18, 1953, Serial No. 375,060

Claims priority, application Germany August 22, 1952 2 Claims. (Cl.41-42) This invention relates to the glossing of aluminum and aluminumalloys as well as of articles made thereof or being coated with aluminumor aluminum alloys.

Many processes have been developed to serve this purpose, which areeither based on a purely chemical basis or on the use of electriccurrent.

In the chemical processes of this type, which have the advantage of asmaller apparatus expenditure and of a simpler manipulation over theelectrical or electrolytical procedures, alkaline or acid solutions areused, which remove a portion of the metal surface and level the same;the glossing capacity of these methods is based upon this removal andupon the leveling action.

By using for this purpose solutions containing nitrate, hydrogen,ammonium, fluorine and advantageously also lead ions it was discovered,by the inventor, that the gloss and the brilliancy of the treatedsurfaces may be improved in a surprisingly effective manner if, togetherwith the glossing agents, substances are used which inhibit the glossingaction of the said substances. These substances exert an inhibitinginfluence upon the gloss producing agents and solutions; in thefollowing they will be denoted as inhibitors." Moreover, it was foundthat by the use of these inhibitors, in spite of the surprising surfaceleveling action, the surface removal of the treated aluminum or alloy isreduced and that therefore a saving results of metal and glossingsolution. Moreover by the application of the inhibitors a reflectivecapacity of 82 per cent may be obtained of the treated surfaces. Theweight reduction of the treated articles amounts only to 24 grams persquare meter, corresponding to a thickness reduction of of a millimeter;in the same bath and without an inhibitor, a weight reduction isproduced of 54 grams per square meter corresponding to a reduction ofthe thickness of of a millimeter. The highest reflective capacityobtainable without the inhibitors amounts to 79.8%.

Consequently, by the surface treatment according to the invention thedimensional stability of the worked articles may be satisfactorilypreserved; this is particularly true in view of the following protectiveoxidation of the glossed articles whereby a thickness reduction of, forinstance, of a millimeter can be fully compensated.

Substances are preferably used as inhibitors which reduce or slow-downthe surface attacking action of the gloss producing baths without,however, creating protective layers. Good results are obtained by theadmixture to the gloss or brilliancy creating solutions of arable gum,dextrin, gelatin, glue, lignin sulfo acid and salts thereof, artificialtanning substances, nicotine, nicotine sulfate, amines such as amylamine and also dibenzyl sulfide.

It is advisable to add these inhibitors to the gloss creating solutionsat a rate of about 0.5 to 2 per cent and preferably 1 per cent. Thebeneficial action of these substances becomes particularly apparent if alead ion concentration is maintained in the glossing solutions amountingto 0.05 to 0.5 gram per liter.

The gloss producing solution may, for instance, contain per one liter1.6 to 2.7 and preferably 2.1 gram mol. nitric acid 1.8 to 3.5 andpreferably 2.8 gram mol. ammonium 3.5 to 7.0 and preferably 5.6 grammol. hydrofluoric acid 2.5 times 10- to 2.5 times 10 gram mol. lead 0.5to 2 per cent and preferably 1.0 per cent of an inhibitor.

The treating bath may be produced by mixing a suitable quantity of purenitric acid, pure hydrofluoric acid and preferably pure lead nitrate andadding upon admixture of the inhibitor an aqueous ammonia solution or byentering gaseous ammonia into the diluted solution.

By the use of these weak, thinly fluid and therefore cheap solutions itis possible, after a short treating time, to produce high gloss surfaceson aluminum and its alloys and particularly on magnesium containingaluminum. The displacement, preferably manually, of the articles in thetreating solutions is advisable as it prevents the adherence of gasbubbles to the same.

The invention is particularly applicable to articles made of purehomogeneous aluminum and aluminum magnesium alloys having a maximumcontent of 2.7 per cent Mg or articles consisting of other materials andbeing ovated with aluminum or the said alloys.

Moreover, it has been found that colorless transparent protective layersdo not reduce the gloss of the treated Al articles if the latter areanodically or chemically oxidized after the glossing treatment.

The invention has the advantage of far greater simplicity and economyover the known combined polishing and electrolytical treatments; thisexperience applies particularly to small mass-produced articles, forinstance, casings for condensors, bottle capsulae, stamped objects andthe like.

A protective layer may also be produced in conformity with the inventionupon reflectors without in the least reducing their shine.

Moreover, and in conformity with the invention articles made of othermaterials than aluminum may be effectively glossed by covering the sameentirely or partly with a thin layer or a foil of pure aluminum havingpreferably a purity of 99.8 to 99.95 per cent and treating them aspreviously described. The aluminum foils may contain up to about 2.7 andpreferably 0.5 per cent Mg thereafter, the treated articles may bechemically or anodically oxidized.

The articles to be treated in the above described manner may consist ofsynthetic materials and may be made, for instance, by injection molding;therefore the invention may be advantageously used for the production ofvehicle parts and fittings. The portions of the articles not coveredwith Al must be provided with a protective coating prior to theirlocation into the glossing solution and prior to oxidation.

A plastic sealing or binding material may be inserted between thearticles and the coating to be glossed which material may be hardenableby chemical action or heat, whereby a satisfactory cohesion is producedbetween the article and the coating.

It may be advantageous to replace the plastic sealing materials by paperor similar soft materials which are impregnated with binders. In thismanner the influence of unevenness of the article surface onto thecoating is eliminated.

The layers created by oxidation on aluminum or its alloys are boiled inthe usual manner in water to produce a tight seal.

This procedure, however, causes, particularly with oxidized articelsmade of very pure aluminum, an undesirable change of the surface of theoxidized layers, which are thereby softened. In this state the surfaceis not resistant to abrasion; moreover, upon abrasion a cloudiness isproduced in the lower and harder portion of the oxidized layer becauseparts of the soft or upper layer are pressed into the pores of theharder layers located underneath the upper softer layers which cannot beremoved.

This disadvantage is fully eliminated by a treatment of the oxidizedparts with an aqueous ammonium bifluoride and alkalichromate containingsolution. Solutions are successfully usable which contain by weight 0.3to 0.9 and preferably 0.6 per cent NHs, 0.7 to 2.1 and preferably 1.4percent HP, 0.6 to 1.8 and preferably 1.2 percent CF03 and which may beproduced by mixing 2 kilograms of a 70 percent hydrofluoric acid, 2.4kilograms ol :1 25 percent ammonia solution and 4 kilograms sodiumchromate in l liters water. The duration of the trcatment is to be atleast 12 minutes.

These solutions may be generally replaced by solutions which act as asolvent upon the hydrolised and compacted layers, whereas the lesshydrolised layers remain less influenced. For instance, nitric acidpreferably having a concentration of between 25 to 40 percent preferably30 percent, by weight or a mixture of a solution of nitric acid andchromic acid may be used, the treatment being conducted at a temperatureof above 20 C. and partic ulariy 20 to 25 C. for a period of l to 5minutes, preferably 3 minutes.

The invention will now be more fully described by the followingexamples.

Example 1 A workpiece made of a pure aluminum and containing 0.5 percentMg is subjected for a period of sec ends and at a temperature of 50 C.to the action of an aqueous bath containing 13 weight percent of nitricacid, 16 weight percent of ammoniumbifluoride, 0.02 weight percent oflead nitrate and 1 weight percent of a guru arabic solution the latteracting as the glossing inhibitor. The surface of the thus treatedaluminum article has a reflective capacity of 82 percent at a Weightreduction of 24 grams per sq. meter and a thickness reduction of imianmillimeter.

By an identical treatment without the admixture of the inhibitor thevalues of the reflective capacity were 79.8 percent and the weightreduction 54 grams per square meter and a l millimeter thicknessreduction.

Example 2 Condensator housings were exposed to the action of an aqueoussolution which contained 2.l gram/mol. nitric acid 2.8 gram/mol.ammonium 5.6 gram/mol. hydrofluoric acid 1% by weight of arabic gum perliter at a temperature of l6 C. and a time of seconds.

After rinsing with water, the articles were exposed to the action of asolution containing per liter 047 gram mol. sodium carbonate 0.047gram/mol. sodium chromate 0.0008 gram/mol. sodium silicate for tenminutes at a temperature of 95 to 100 C.

An entirely colorless, pervious, bright protective surface layerresulted with increased chemical stability and grip resistance.

Example 3 Reflectors made of pure aluminum sheets having a thicknes of0.8 mm. and an A] content of 99.9 per cent were treated for theobtainment of a high reflective capacity as follows:

Removing of fat from the surface by dipping the sheets at a temperatureof 25 C. in a solution consisting of 80 volume parts of concentratednitric acid, and 20 volume parts of concentrated hydrofluoric acid.

These reflectors were dipped in a brilliancy promoting bath at atemperature of 58 C. for 10 seconds, which bath contained 2.3 gram/mol.of nitric acid 3 gram/mol. of ammonium 6 gram/mol. of hydrofluoric acidtimes 10* gram mol. lead it] grams arabic gum.

Hereupon an intermediary treatment was carried on for seconds with a percent nitric acid. The follow ing step was an additional shining of thereflectors in an electrolyte containing grains of trisodiumphosphate andlit) grams of sodium carbonate per liter at a median current density of4 amp/dm. for 10 minutes.

Tea pots made of a homogeneous pure aluminum magnesium alloy sheethaving 0.5 per cent Mg and a thickness of 15 millimeters by drawing andhaving a handle attached thereto of a thickness of 10 millimeters andmade of the same alloy are treated by the following steps:

Removal of fat by dipping the articles in a solution which containsvolume parts of concentrated nitric acid, and 20 volume parts ofhydrofluoric acid at 25 C. for 10 minutes;

Heating the fat-freed pots in hot water having a temperature of l00 C.for at least 5 minutes whereby the pot and the handle are heated to anequal temperature;

lossing the hot pots by moving the same for 15 seconds at a temperatureof 60 C. through a solution, which contained per liter 2.1 gram mol.nitric acid 2.8 gram mol. ammonium 5.6 gram mol. hydrofluoric acid 25times 10- gram mol. lead 10 grams arabic gum.

An intermediary treatment of the pots consists of a ten second dippinginto a 25 per cent nitric acid at 25 C. and thereupon suspending of thepots at 20 C. in a solution containing .06 gram mol. sodium chromate forat least 10 seconds.

Electrolytic oxidation of the treated pots in a sulfuric acidelectrolyte at a temperature of 20 C. and a current density of 35amp/min.

Subsequent packing of the oxidic layer by handling the pots in distilledwater at a temperature of 92 C. for 20 minutes.

After-treatment of the shined and anodically oxidized pots by boilingthe same in distillated water and handling the same in a 25 weight percent nitric acid at a temperature of 25 C. for two minutes.

Rinsing with water between the individual treating steps.

Example 5 vol. parts of concentrated nitric acid, and 15 vol. parts ofconcentrated hydrofluoric acid.

Heating the handles in water at 100 C. for 3 minutes until the same havereached a temperature of 60 C.

Glossing the heated handles for 15 seconds by a to-andfro movementthrough a solution at 50 C. which contained 2.1 gram mol. nitric acid2.8 gram mol. ammonium 5.6 gram mol. hydrofluoric acid 25 times 10" grammol. of lead, and 50 grams of dextrin.

An intermediary treatment by dipping the handles in 30 weight per centnitric acid for 30 seconds and suspension of the same at 22 C. in asolution which contained 0.12 gram mol. per liter of sodium chromate forat least minutes.

Anodic oxidation of the handles in a sulfuric acid elec trolyte at 18 C.and a current supply of 50 amp/min.

Compaction of the anodic protective layer applied to the handles bysuspension of the same in distilled water having a temperature of 90 to92 for 30 minutes.

Final treatment of the handles in a 40 per cent nitric acid at atemperature of 25 C. for two minutes.

Rinsing with water between the individual treating steps.

Example 6 Decorative buttons made of a homogeneous pure aluminum alloywith 1 per cent Mg are treated by the following steps:

Glossing for 10 seconds by a to-and-fro movement at 52 in a bathcontaining per liter.

2.1 gram mol. nitric acid 2.8 gram mol. ammonium 5 .6 gram mol.hydrofluoric acid 25 times 10' gram mol. lead, and 10 grams arabic gum.

An intermediary treatment for 10 seconds consisting in dipping thebuttons in a 40 per cent nitric acid at a temperature of 22 C.

Application to the buttons of a protective layer by chemical treatingconsisting in dipping the same for 10 minutes at a temperature of 95 C.in a solution which contained per liter 0.47 gram mol. sodium carbonate0.047 gram mol. sodium chromate 0.0008 gram mol. sodium silicate.

Rinsing with water between the individual treating steps. Since certainchanges in carrying out the above process could be made withoutdeparting from the scope thereof, it is intended that all mattercontained in the above description shall be interpreted as illustrativeand not in a limiting sense.

It will be apparent to those skilled in the art that the novelprinciples of the invention disclosed herein in connection with specificexemplifications thereof will suggest various other modifications andapplications of the same.

It is accordingly desired that in construing the breadth of the appendedclaims they shall not be limited to the specific exemplifications of theinvention described herein.

Having thus described the invention, what I claim as new and desire tobe secured by Letters Patent, is as follows:

1. A method for brightening the surface of articles made of aluminum andof aluminum alloys and particularly magnesium containing aluminum alloyscomprising applying to said articles a hot aqueous solution containingper liter between 1.6 to 2.7 gram molecules of nitric acid, between 1.8to 3.5 gram molecules of ammonium, 3.5 to 7.0 gram molecules ofhydrofluoric acid, 2.5 times l0- to 2.5 times 10' gram molecules leadnitrate and 0.5 to 2 percent of an inhibitor selected from a groupconsisting of gum arabic and dextrin.

2. A method for brightening the surface of articles made of aluminum andof aluminum alloys and particularly magnesium containing aluminum alloyscomprising applying to said articles a hot aqueous solution containingper liter between 1.6 to 2.7 gram molecules of nitric acid, between 1.8to 3.5 gram molecules of ammonium, 3.5 to 7.0 gram molecules ofhydrofluoric acid, 2.5 times 10 to 2.5 times 10* gram molecules leadnitrate and about 1 percent of an inhibitor selected from a groupconsisting of gum arabic and dextrin.

References Cited in the file of this patent UNITED STATES PATENTS2,593,447 Hesch Apr. 22, 1952 2,593,449 Hesch Apr. 22, 1952 2,614,912Rice Oct. 21, 1952 2,614,913 Reindl et al. Oct. 21, 1952 2,625,468Prance et al -7 Jan. 13, 1953 2,650,156 Shelton-Jones Aug. 25, 19532,650,875 Dvorkovitz Sept. 1, 1953 OTHER REFERENCES Lange Handbook ofChemistry, fourth edition, Handbook Publishers, Sandusky, Ohio, 1941,pages 946 and 947.

1. A METHOD FOR BRIGHTENING THE SURFACE OF ARTICLES MADE OF ALUMINUM ANDOF ALUMINUM ALLOYS AND PARTICULARLY MAGNESIUM CONTAINING ALUMINUM ALLOYSCOMPRISING APPLYING TO SAID ARTICLES A HOT AQUEOUS SOLUTION CONTAININGPER LITER BETWEEN 1.6 TO 2.7 GRAM MOLECULES OF NITRIC ACID, BETWEEN 1.8TO 3.5 GRAM MOLECULES OF AMMONIUM, 3.5 TO 7.0 GRAMS MOLECULES OFHYDROFLUORIC ACID, 2.5 TIMES 10-5 TO 2.5 TIMES 10-4 GRAM MOLECULES LEADNITRATE AND 0.5 TO 2 PERCENT OF AN INHIBITOR SELECTED FROM A GROUPCONSISTING OF GUM ARABIC AND DEXTRIN.